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DENTAL METALLURGY. 1 99

Atomic Weight.—The weight of an atom of an element,
as compared with the weight of an atom of hydrogen.
Alloys.—The mechanical properties of metals are very
much clianged by associating them with each other in the
form of an alloy.* They are usually harder, more brittle,
/c\s\s- ductile and tenacious, their power for conducting heat
and electricity being greatly reduced. Their fusing point
is usually lowered, that is, the alloy melts more readily than
that of the least fusible constituent metal, and oftentimes
lower than that of any of the constituent metals. This
will be considered more fully in another place.
Noble and Base Metals.—It is customary in studying
metals to classify them into noble and base metals.
The first, or noble metals are those having a very feeble
affinity for oxygen, and are capable of being readily sepa-
rated from any combinations with oxygen by the applica-
tion of moderate heat. This lack of affinity for oxygen
renders them incapable of rusting or tarnishing by oxida-
tion in the air.
The second class or base metals, are those which lose their
metallic luster, oxidize, or rust in the air at ordinary tem-
peratures. The term *' noble " is therefore applicable to
gold, silver, platinum and the rare metals found associated
with platinum. Mercury is also included as one of the
noble metals.
Native Metals.—A few of the metals are sometimes
found native, that is, in a free or uncombined state. Us-
ually, however, the}^ are found in combination wnth non-
metallic elements, such as oxygen, sulphur, etc. ; these are
known as ores.
Ores.-—As has been indicated, ores are metals found in
nature combined with non-metallic elements forming a

* An alloy is a mixture or compound of two or more metals, usually
formed by fusing the metals together. See chapter upon the subject.